122-28176 Parallax Inc, 122-28176 Datasheet - Page 238

• Manufacturer Part Number: 122-28176
• Description: GUIDE STUDENT PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 122-28176

• Accessory Type: Manual
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Propeller Education (PE) Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Page 228 · Process Control
the half-watt resistor also is small. As a result, the overshoot of your system will probably
only be several degrees.
When the temperature does turn around and begins to fall, as it passes the setpoint, the
heater is once again turned on. Undershoot may occur for similar reasons as did the
overshoot. During the time the heater is coming up in temperature, the ambient
temperature has continued downward. Continuous cycling above and below the desired
setpoint is typical of on-off control. The rate of this cycling and the degree of overshoot
depends on the characteristics of the system. On-off control is suitable for processes that
have large capacity, can tolerate sluggish response, and sustain a relatively constant level
of disturbance. If our incubator were large, well insulated, and kept in a constant room
environment, on-off control would be acceptable. After the process has had a chance to
cycle a few times, note the minimum and maximum overshoot values in the minimum
and maximum temperature boxes.
A major problem with on-off control is that the output drive may cycle rapidly as the
measurement hovers about the setpoint. Noise riding on the analog sensor measurement
would be interpreted as rapid fluctuation above and below the setpoint. The timing
diagram in Figure 7-4 represents this problem.
Figure 7-4 Effects of Noise on Signal Causing Heater Cycling